For example, a conventional method for producing a tetrafluoroborate includes a method for producing lithium tetrafluoroborate, which typically includes allowing lithium carbonate to react with a fluoroboric acid solution to obtain lithium tetrafluoroborate. In this method, the resulting salt is lithium fluoroborate monohydrate, which is represented by the formula LiBF4.H2O, and therefore, dehydration by heating at about 200° C. is necessary. Unfortunately, heating at about 200° C. can decompose lithium tetrafluoroborate and therefore may reduce its purity. In addition, several thousand ppmw of water also remains. Therefore, this production method is not necessarily satisfactory in view of the controllability of the reaction and the purity of the resulting product and so on.
To solve this problem, for example, Patent Document 1 listed below discloses a method of producing lithium tetrafluoroborate by a process including blowing boron trifluoride gas into a lithium fluoride-containing, non-aqueous, organic solvent for a lithium secondary battery electrolyte to allow the lithium fluoride to react with the boron trifluoride.
In the above production method, however, the organic solvent forms a suspension (slurry) because lithium fluoride has low solubility in the organic solvent. In the production process, therefore, it is difficult to circulate the lithium fluoride-containing organic solvent, which causes a problem in which it is difficult to produce a tetrafluoroborate by a continuous process.